Who do you think is the strongest person he can defeat with this incredible gear?

Darkseid probably maybe even the Anti-monitor.

If the Godslayer sword was made to specifically target someone, then pretty much anyone that isn’t nigh -omnipotent. Hephaestus was tossing sups and wonderwoman around easily, yet the weapon made quick work of him.

Slade sparred Superman and did pretty well but the weapon wasn’t after sup’s blood. It depends completely on if he gets the full power of the blade aiding him or not. His opponent also needs to deal blunt force damage only.